Method of selectively contolling the self-sealing ability of a tyre and self-sealing tyre for vehicle wheels

ABSTRACT

A method of selectively controlling the self-sealing ability of a tyre obtained by application of a sealing assembly on a drum and subsequent formation thereon of a precursor of a green tyre inclusive of liner and carcass, followed by shaping, curing and molding. The sealing assembly includes: a self-supporting thermoplastic film of polyamide or polyester and a layer of sealing material possessing viscoelasticity and stickiness features, which is associated with and supported by the self-supporting thermoplastic film. The sealing assembly is easily puncturable by a sharp-pointed element while it maintains such a deformability and stickiness that it helps in the transfer of the sealing material during ejection of the sharp-pointed element and limits the transferred material amount to such an extent that holes bigger than a predetermined value are not sealed.

The present invention relates to a method of selectively controlling theself-sealing ability of a tyre and to a self-sealing tyre for vehiclewheels.

A tyre for vehicle wheels generally comprises a carcass structureassociated with a belt structure. A tread band is applied at a radiallyexternal position to the belt structure. The green tyre is build throughassembly of respective semi-finished products of an elastomeric blend.

After building of the green tyre carried out by assembly of respectivecomponents, a curing and molding treatment is generally carried outwhich aims at determining the structural stabilisation of the tyrethrough cross-linking of the elastomeric compositions and also atimpressing it with a desired tread pattern and possible distinctivegraphic marks at the sidewalls.

Self-sealing tyres are known that are able to delay or prevent theescape of air and the consequent tyre deflation due to a puncture causedby a sharp-pointed object (a nail, for example). In order to obtain thisresult, a self-sealing tyre comprises at least one layer of sealingpolymeric material that can adhere to the object inserted therein andcan further flow into the puncture or hole when this object is removed,thereby sealing the hole itself and preventing the escape of air fromthe tyre. This material inside the finished (cured and molded) tyre mustbe deformable and sticky.

Document EP 1 435 301 discloses a self-sealing tyre obtained bydisposing an elastomeric composition, containing 0.2 to 20 parts byweight of a peroxide based on 100 parts by weight of an elastomercontaining not less than 50% by weight of polyisobuthylene, at an innersurface of an uncured tyre. In an embodiment, a film of thermoplasticresin is disposed on a radially internal side of the sealing layer. Thethickness of the thermoplastic-resin film is included between 0.05 mmand 0.3 mm. The elastomeric composition is heated during the tyrecuring, causing a reaction involving decomposition of thepolyisobutylene and formation of an elastomeric sealing layer.

According to the present invention it has been found that self-sealingof the tyre in operation is not a mere problem of sealing a hole causedby a sharp-pointed element. It is the Applicant's perception thatself-sealing is a problem of selective control of the sealing itself, inorder to exclude the repair of punctures having the effect of weakeningthe tyre structure.

According to the present invention, the problems connected withself-sealing are not mainly those of ensuring a long running but ratherthose of ensuring a safe use of the punctured and self-repaired tyre.

To the aims of this invention it has been seen that an uncontrolled orunselective self-repair runs the risk of giving the vehicle's driver afalse feeling of safety that will involve dangers in the vehiclehandling.

In particular, the Applicant has perceived that, should thesharp-pointed element have produced a puncture of big sizes and at allevents beyond a given predetermined diameter, this puncture might havecaused the structural weakening of the tyre to such an extent thatself-sealing of the puncture, on ejection of the sharp-pointed elementfrom the tyre, could not enable the driver to become aware of the extentor importance of the damage suffered by the tyre. Consequently, underthis situation, the tyre could no longer ensure reliability andcontinuity in the performance of use. For instance, it is theApplicant's opinion that punctures produced by sharp-pointed elements ofan 8 mm diameter or higher can cause structural damages to the tyrecapable of jeopardising a safe running.

The Applicant has found that it is possible to selectively control theself-sealing ability of a tyre mounted on a vehicle wheel and puncturedby a sharp-pointed element such as a nail or the like, in particularwhen said nail exits the tyre, through a sealing assembly comprising alayer of a predetermined thickness of sealing material in associationwith a thermoplastic thin film. Said thermoplastic film has structuralfeatures involving support of said sealing material, resistance to thetemperatures typical of curing and sufficient viscoelasticity to enableit to become deformed during shaping and during curing/molding of thetyre.

Preferably, the thermoplastic thin film also has air-tightness featuresand/or easy pierceability features in case of puncturing by asharp-pointed object.

More specifically, in a first aspect, the present invention relates to amethod of selectively controlling the self-sealing ability of a tyre,said method including:

-   -   arranging a sealing assembly on a drum and, subsequently,    -   forming a precursor of a green tyre inclusive of at least one        liner and a carcass on said sealing assembly and subsequently,    -   shaping, moulding and curing the combined sealing complex and        tyre precursor, such a sealing assembly comprising:

i. a self-supporting thermoplastic film of polyamide or polyester;

ii. a layer of sealing material possessing viscoelasticity andstickiness features, which is associated with and supported by saidself-supporting thermoplastic film;

said sealing assembly being easily puncturable by a sharp-pointedelement while maintaining such a deformability and stickiness that itcontributes to transfer of the sealing material during ejection of thesharp-pointed element and limits the transferred amount thereof to suchan extent that holes bigger than a predetermined value are not sealed.

In a second aspect, the present invention relates to a self-sealing tyrefor vehicle wheels, comprising:

-   -   at least one carcass ply, a tread band applied at a radially        external position to said carcass ply at a crown region, at        least one liner applied at a radially internal position to said        carcass ply, a sealing assembly applied at a position radially        internal to the liner and axially extending at least over the        whole crown region of the tyre;

wherein said sealing assembly comprises a self-supporting thermoplasticfilm of a polyamide or polyester material, a layer of sealing materialassociated with and supported by said self-supporting thermoplasticfilm;

wherein the self-supporting thermoplastic film is radially internal tothe layer of sealing material and said layer of sealing material isplaced directly in contact with the liner;

wherein said self-supporting thermoplastic film has a thickness smallerthan 50 μm;

wherein said sealing layer has a maximum thickness smaller than about 6mm.

Said assembly is placed in the radially innermost position of the tyreso as to cover the whole circumference thereof over a radial (ortransverse) extension interesting at least all the crown region thereof.

The tensile features (to be expressed within the shaping andcuring/molding scope) and the thickness of the self-supportingthermoplastic film, as well as the viscoelasticity and stickinessfeatures of the sealing material and the thickness of same must ensurethat the assembly will react to the exit of the puncturing element by aselective sealing action avoiding punctures to be sealed when they arebigger than a predetermined value in relation to the sizes of the tyreand the tyre destination of use.

The sealing ability of the material referred to in the present inventionis in relation both with the viscoelastic features and stickinessthereof: the former enables flowing of the material inside the punctureand the latter gives the material the dragging force that is exertedthereon by contact with the sharp-pointed element in movement.

Within the scope of the present invention, the assembly must be easilypierceable by a sharp-pointed element puncturing the tyre, whilemaintaining a flexibility adapted to help in controlling transfer of thesealing material once the sharp-pointed element is ejected from thetyre, in particular in use.

Specifically, the Applicant's experiences show that at the moment thesharp-pointed element punctures the tyre, the punctured sealing assemblywill form, by eversion towards the inside of the tyre, a sort of bulgearound the puncturing element. The same experiences show that thepunctured radially-innermost layer (thin thermoplastic film) of theassembly, substantially follows the puncturing element, together withthe sealing material, during ejection thereof, being turned inside outthereby forming a sort of crater with inner concavity. As a result oftests carried out by the Applicant, it appears that a thinnerthermoplastic film is not so capable of following the flow of thesealing material during ejection of the sharp-pointed element. It iseven believed that the thin film since it is deformable, exerts acontrol on the maximum amount of sealing material following thesharp-pointed element during ejection of the latter, this beingconnected with the thickness of this sealing material and theviscoelasticity and stickiness qualities of said material.

The present invention, in at least one of the above aspects, can exhibitone or more of the preferred features hereinafter described.

Preferably, said self-supporting thermoplastic film is made of apolyamide selected from: nylon 6, nylon 66, nylon 46, nylon 11, nylon12, nylon 610, nylon 612, nylon 6/66 copolymer, nylon 6/66/610copolymer, nylon MXD 6, nylon 6T, nylon 6/6T copolymer, nylon 66/PPcopolymer, nylon 66/PPS copolymer, used alone or in combination thereof.

Preferably, said self-supporting thermoplastic film is made of apolyester selected from: polybutylene terephthalate (PBT), polyethyleneterephthalate (PET), polyethylene isophtalate (PEI), polybutyleneterephthalate/tetramethylene glycol copolymer, PET/PEI copolymer,polyarylate and polybutylene naphthalate.

Preferably, the self-supporting thermoplastic film has an elongation atyield greater than 5%.

Preferably, the self-supporting thermoplastic film has an elongation atyield not exceeding 30%.

Preferably, the self-supporting thermoplastic film has a yield strengthcomprised from about 20 MPa to about 60 MPa, evaluated following theASTM D882 standard.

Preferably, the self-supporting thermoplastic film has an ultimateelongation greater than 70%.

Preferably, the self-supporting thermoplastic film has an ultimatetensile strength comprised from about 20 MPa to about 150 MPa.

To the aims of the present invention, the elongation and strengthfeatures are evaluated by a tensile stress test carried out followingthe ASTM D882 standard.

According to the Applicant's experiences, after shaping, thethermoplastic film remains in a deformed state imposed by the newgeometry. The film reaction to this imposed deformation is of theviscoelastic type: that is to say, a stress is produced that counteractsdeformation and decreases in time. The Applicant has perceived that thisstress decrease must be quick to prevent an excessive elastic stressfrom operating in such a manner as to deform the green tyre or fromcausing separation of the radially internal layers.

Preferably, the self-supporting thermoplastic film has such a stressrelaxation referred to time, that in the 10 first seconds followingachievement of an imposed predetermined elongation the thermoplasticfilm shows a reduction of at least 20% of the stress necessary formaintaining such an imposed elongation.

Preferably, the self-supporting thermoplastic film has such a stressrelaxation referred to time that within the 300 seconds followingachievement of an imposed predetermined elongation the thermoplasticfilm shows a reduction of at least 35% of the stress necessary formaintaining such an imposed elongation.

To the aims of the present invention, the term “stress relaxation”refers to the stress or load/strength measured after a thermoplasticfilm sample has been elongated to speeds in the order of those carriedout during shaping (500 mm/min, for example), up to a predeterminedelongation value, and maintained to such a length for a predeterminedtime, 5 minutes (300 s), for example. The stress relaxation with animposed deformation is expressed as the percent load reduction relativeto the starting load, the predetermined elongation in the predeterminedtime being maintained.

The stress relaxation is determined by calculating the percent reductionof the starting load measured after elongation of the sample at a 500mm/min speed up to the predetermined length between 110% and 150%, thestarting length being considered as 100%, the sample being maintained tosuch a length and said reduction being measured at 10 seconds and 300seconds.

Preferably, said self-supporting thermoplastic film before shaping has athickness smaller than 50 μm.

Preferably, said self-supporting thermoplastic film before shaping has athickness comprised from about 10 μm to about 30 μm.

During building of the tyre carcass, first the sealing assembly isapplied on the building drum according to an annular form obtained by ajoint at the ends of the sealing assembly. The following toroidalconformation of the carcass structure and the sealing assembly radiallyinternal thereto involves a radial expansion both of the layer ofsealing material and of the self-supporting thermoplastic film. Theabove mentioned size and material features relating to theself-supporting thermoplastic film are of such a nature that theelastoplastic deformation is allowed without the film or joint beingbroken, during the tyre shaping. The sealing assembly therefore must beable to follow the extension and shape of such a conformation. The abovementioned features of said film are suitable to keep it unimpaired (thefilm does not melt) during curing of the tyre.

The above mentioned features of said film in addition enable it to bearits own weight and that of the sealing layer disposed thereon withoutbecoming deformed during transport of the sealing assembly, to thebuilding drum on which it has to be applied, for example. Thesealing-material layer in fact becomes deformed under its own weight ifit does not rest on a support.

Preferably, said layer of sealing material before shaping has athickness smaller than about 6 mm.

Preferably, said layer of sealing material before shaping has athickness greater than about 3 mm.

Preferably, the sealing material comprises:

-   -   40 phr to 80 phr, preferably 50 phr to 70 phr, of a synthetic or        natural elastomer material;    -   20 phr to 60 phr, preferably 30 phr to 50 phr, of an elastomeric        block copolymer, preferably a styrene-butadiene elastomer (SBR);    -   40 phr to 60 phr, preferably 50 phr to 60 phr, of process oil;    -   15 phr to 60 phr, preferably 20 phr to 40 phr, of at least one        tackiness agent; and    -   1 phr to 40 phr, preferably 5 phr to 30 phr, of at least one        reinforcing filler.

The composition and thickness of the sealing-material layer areselected, preferably within the above mentioned features, in relation tothe type of tyre to be produced so as to supply the optimalviscoelasticity and stickiness features for each use condition of thetyre. In fact, the Applicant took care to apply the invention to tyresfor four-wheeled vehicles for use on the road, such as tyres adapted toequip medium- and high-powered cars for people transport (measures ofthe chord 195 mm to 245 mm), but without any prejudice the invention isalso suitable for tyres of small runabouts or for high-performance (HP)and ultra-high-performance (UHP) tyres, the chord of which measures 145mm to 295 mm, for example. If the appropriate adjustments are made, thepresent invention can apply to tyres for different vehicles such asmotorcycles or heavy transport means for persons and things.

Preferably, in the finished tyre, said self-supporting thermoplasticfilm has a thickness smaller than about 25 μm.

Preferably, in the finished tyre, said self-supporting thermoplasticfilm has a thickness greater than about 5 μm.

Preferably, in the finished tyre, said self-supporting thermoplasticfilm has a resistance to puncturing lower than about 30N.

Preferably, in the finished tyre, said self-supporting thermoplasticfilm has a resistance to puncturing lower than about 15N, morepreferably lower than about 10N.

To the aims of the present invention, the resistance to puncturing isevaluated by a test carried out following the ASTM F1306 standard.

Preferably, in the finished tyre, said layer of sealing material has amaximum thickness greater than about 2.5 mm.

The optimal selective sealing of the sealing assembly according to theinvention has been obtained with the film parameters pointed out above.Operation of this assembly is not known. It was found by chance, in asuccessful example thereof, when a nylon film of a 200 micron thicknesswas replaced with a 18 micron film. In fact, before the presentinvention, it was believed that a film thickness lower than 50 micronswould have not allowed the material to be sealed due to the probablebreaking of the film carrying the sealing material. Having found thecontrary was a surprise for the Applicant who so far is unable toexplain the mechanism with certainty, but can only make reference tothat which was observed during the tests.

Preferably, the tyre comprises two elongated elements of elastomericmaterial each applied at a respective circumferential edge of thesealing assembly. Each elongated element of elastomeric material has anaxially internal portion disposed radially internal to the sealingassembly and in contact with said sealing assembly and an axiallyexternal portion disposed in contact with the liner.

Preferably, the layer of sealing material has an axial extension greaterthan the axial extension of the self-supporting thermoplastic film andwherein each elongated element of elastomeric material is directly incontact with said layer of sealing material and said self-supportingthermoplastic film.

The elongated elements ensure side adhesion of the sealing assembly tothe liner.

The axially opposite edges of the self-supporting thermoplastic film aremaintained sticking to the sealing material by overlapping of eachelongated element adhering to the respective side end of the sealingmaterial itself.

The lateral elongated elements of elastomeric blend contain the materialduring shaping and curing of the tyre, when the inner pressure of themold presses the carcass against the inner walls of the mold itself.

In addition, the elongated elements during shaping of the tyre preventthe self-supporting thermoplastic film from separating from the sealingmaterial at the axially opposite circumferential edges thereof.

Further features and advantages will become more apparent from thedetailed description of a preferred but not exclusive embodiment of amethod of selectively controlling the sealing of punctures in aself-sealing tyre for vehicle wheels and of a self-sealing tyre forvehicle wheels in accordance with the present invention.

This description will be set out hereinafter with reference to theaccompanying drawings, given by way of non-limiting example, in which:

FIG. 1 diagrammatically shows a radial half-section of a self-sealingtyre for vehicle wheels;

FIG. 2 shows a sealing assembly associated with two elongated elementsof elastomeric material and designed to form part of the self-sealingtyre seen in FIG. 1.

Identified in FIG. 1 with reference numeral 1 is a self-sealing tyre forvehicle wheels which generally comprises a carcass structure 2 includingat least one carcass ply 3 having respectively opposite end flaps inengagement with respective annular anchoring structures 4, possiblyassociated with elastomeric fillers 4 a, integrated into the regions 5usually identified as “beads”. The carcass ply 3 comprises a pluralityof textile or metallic reinforcing cords disposed parallel to each otherand at least partly coated with a layer of elastomeric material.

Associated with the carcass structure 2 is a belt structure 6 comprisingone or more belt layers placed in radial superposed relationshiprelative to each other and to the carcass ply 3 and havingtypically-metallic reinforcing cords. These cords can have a crossedorientation relative to the circumferential extension direction of tyre1.

A tread band 7 of elastomeric blend, like other semifinished productsconstituting tyre 1, is applied at a radially external position to thebelt structure 6.

In addition, respective sidewalls 8 of elastomeric blend are applied atan axially external position to the side surfaces of the carcassstructure 2, each extending from one of the side edges of the tread band7 until close to the respective annular anchoring structure to the beads5.

A radially internal surface of tyre 1 is further preferably internallycoated with a substantially air-tight layer of elastomeric material, aso-called liner 9.

In the embodiment shown in FIG. 1 tyre 1 is of the type formotor-vehicles.

Typically, in this case the belt structure 6 further comprises at leastone radially external layer comprising textile cords disposed at asubstantially zero angle relative to the circumferential extensiondirection of the tyre.

In accordance with alternative embodiments of the present invention,tyre 1 is of the type intended for use on heavy vehicles. The term“heavy vehicle” is generally understood as indicating a vehiclebelonging to categories M2˜M3, N1˜N3 and O2˜O4 defined in “ConsolidatedResolution of the Construction of Vehicles (R.E.3) (1997)”, Annex 7,pages 52-59, “Classification and Definition of Power-Driven Vehicles andTrailers”, such as lorries, lorries with trailers, tractors, buses, vansand other vehicles of this type. The belt structure in a tyre for heavyvehicles (not shown) typically comprises a belt layer usually known withthe name of “gravel-guard belt” which is the radially outermost layer ofthe belt structure and acts as a protective layer against penetration ofstones and/or gravel into the innermost layers of the tyre structure.Preferably, the belt structure of a tyre for heavy vehicles furthercomprises a reinforcing side strip that can be radially superposed onthe second belt layer, at an axial end thereof. The side stripincorporates a plurality of reinforcing elements, preferablyhigh-elongation metallic cords. Preferably, in addition, an insert isdisposed substantially at the shoulder portion, i.e. the portion wherethe side end of the tread band joins the sidewall. In particular, theinsert has a portion that is substantially interposed in a radialdirection between the belt structure and tread band and a portion thatis substantially interposed in an axial direction between the carcassand sidewall.

In accordance with further embodiments of the present invention, tyre 1is intended for motorcycles. The outline of the cross section of thetyre for motorcycles (not shown) has a high camber as it must ensure asufficient footprint area in all inclination conditions of themotorcycle. A camber is defined by the value of the ratio between thedistance f of the tread centre from the line passing through thelaterally opposite ends of the tread, measured in the equatorial planeof the tyre, and the width C defined by the distance between thelaterally opposite ends of the tread. By tyre with a high camber it isdenoted a tyre the camber ratio (f/C) of which is of at least 0.20.Preferably (f/C) is respectively comprised between 0.20 to 0.5 for arear tyre and 0.35 to 0.6 for a front tyre.

The self-sealing tyre 1 according to the invention further comprises alayer of sealing polymeric material 10 disposed at a crown region oftyre 1 and at a radially internal position relative to liner 9. Thelayer of sealing polymeric material 10 extends over the wholecircumferential extension of tyre 1. The layer of sealing material 10has a maximum thickness “t1” substantially at the equatorial plane “X”of the finished tyre 1, i.e. a cured and molded tyre, and becomesthinner towards the axial ends of the crown region (FIG. 1). Preferably,said maximum thickness “t1” is comprised from about 3 mm to about 6 mm.

The sealing polymeric material for instance comprises 40 phr to 80 phr,preferably 50 phr to 70 phr of a synthetic or natural elastomer, 20 phrto 60 phr, preferably 30 to 50 phr, of an elastomeric block polymer,preferably a styrene-butadiene elastomer (SBR), 40 phr to 60 phr,preferably 50 phr to 60 phr, of process oil; 15 phr to 60 phr,preferably 20 phr to 40 phr, of at least one tackiness agent; and 1 phrto 40 phr, preferably 5 phr to 30 phr, of at least one reinforcingfiller.

According to a preferred embodiment, the sealing material can furthercomprise about 1 phr to about 20 phr of at least one homogenising agent.In a further embodiment, the sealing material can further comprise 0.05phr to 5 phr of at least one peptizer. According to a preferredembodiment, the synthetic or natural elastomer included in the sealingmaterial can be selected from the commonly used elastomeric materialsthat can be cross-linked with sulphur, which materials are particularlysuitable for tyre production, or from the elastomeric polymers orcopolymers with an unsaturated chain, having a glass transition (Tg)temperature generally under 20° C., preferably within the range of 0° C.to 110° C. These polymers or copolymers can be of natural origin or canbe obtained through solution polymerisation, emulsion polymerisation orgas-phase polymerisation of one or more conjugated diolefins, optionallymixed with at least one comonomer selected from monovinylarens and/orpolar comonomers in an amount not exceeding 60% by weight. Theconjugated diolefins generally contain 4 to 12, preferably 4 to 8 carbonatoms and can be for example selected from the group comprising1,3-butadiene, isoprene, 2,3-dimetyl-1,3-butadiene, 1,3-pentadiene,1,3-hexadiene, 3-butyl-1,3-octadiene, 2-phenyl-1,3-butadiene, ormixtures thereof. Particularly preferred are 1,3-butadiene or isoprene.

The polar comonomers that may be possibly used can be for exampleselected from: vinylpiridine, vinylquinoline, acrylic acid andalkylacrylic acid esters, nitriles or mixtures thereof such as methylacrylate, ethyl acrylate, methyl metacrylate, ethyl metacrylate,acrylonitrile or mixtures thereof.

Preferably, the synthetic or natural elastomer included in the sealingmaterial can be for example selected from: cis-1,4-polyisoprene (naturalor synthetic rubber, preferably natural rubber), 3,4-polyisoprene,polybutadiene (in particular polybutadiene with a high 1,4-cis content),possibly halogenated isoprene/isobutene copolymers,1,3-butadiene/acrylonitrile copolymers, styrene/1,3-butadienecopolymers, styrene/isoprene/1,3-butadiene copolymers,styrene/1,3-butadiene/acrylonitrile copolymers or mixtures thereof.

The tackiness agents advantageously used in the present invention can beselected from the group of the hydrocarbon resins having a numericalaverage molecular weight included between several hundreds and severalthousands and providing stickiness when the resin is admixed withnatural or synthetic rubber.

As to the resins, different types of synthetic resins can be used. Saidnumerical average molecular weight (Mn) can be measured followingtechniques known in the art, such as by gel permeation chromatography(GPC). In particular, resins derived from petroleum, phenol-basedresins, carbon-based resins, xylene-based resins and natural resins,rosin-based and terpene-based resins for example, can be used as thetackiness agents.

Examples of commercial products concerning aromatic petroleum-basedresins listed by trademark comprise PETROSIN produced by MITSUI SEKIYUKAGAKU Co., Ltd., PETRITE produced by MIKUNI KAGAKU Co., Ltd.,NEOPOLYMER produced by NIPPON SEKIYU KAGAKU Co., Ltd., and PETCOALproduced by TOYO SODA Co., Ltd.

Examples of phenol-based resins comprise alkylphenol-formaldehyderesins, and rosin-modified derived resins, alkylphenol-acetylene resins,alkylphenol- and terpenephenol-modified resins. Specific examples listedby trademark comprise commercial products such as HITANOL 1502 (producedby HITACHI KASEI Co., Ltd.) which is an alkylphenol novolac resin,RESINA SP-1068 (produced by SI GROUP Inc.) which is anoctylphenol-formaldehyde resin, Escorez® 1102 which is an aliphatictackiness resin (produced by ExxonMobil), and KORESIN (produced by BASFCompany) which is a p-t-butylphenol-acetilene resin.

Examples of carbon-based resins include indene cumarone resins. Specificexamples comprise commercial products, listed by trademark, such asNOVARES C resins (produced by RUTGERS CHEMICAL GmbH) which are syntheticindene cumarone resins (NOVARES C10, C30 and C70, for example).

Examples of xylene-based resins comprise the xylene-formaldehyde resins.

Said tackiness agents can be used alone or mixed together.

At least one reinforcing filler can be advantageously added to thecross-linkable elastomeric composition referred to above, in an amountgenerally in the range of 0 phr to 120 phr, preferably 20 phr to 90 phr.The reinforcing filler can be selected from those commonly used forcross-linked products, in particular for tyres, such as carbon black,silica, alumina, aluminosilicates, calcium carbonate, kaolin andmixtures thereof. Particularly preferred are carbon black, silica andmixtures thereof.

According to a preferred embodiment, said carbon black reinforcingfiller can be selected from those having a surface area at least aslarge as 20 m2/g (determined by STSA—Statistical Thickness SurfaceArea—according to ISO 18852:2005).

At a radially internal position relative to the layer of sealingpolymeric material 10 and directly in contact with said layer of sealingpolymeric material 10, a polyamide or polyester self-supportingthermoplastic film 11 is disposed. The self-supporting thermoplasticfilm 11, like the layer of sealing polymeric material 10, extends overthe whole circumferential extension of tyre 1 and has a width or axialextension slightly less than the axial extension of said layer 10.

Preferably, said self-supporting thermoplastic film 11 is a polyamideselected from: nylon 6, nylon 66, nylon 46, nylon 11, nylon 12, nylon610, nylon 612, nylon 6/66 copolymer, nylon 6/66/610 copolymer, nylonMXD 6, nylon 6T, nylon 6/6T copolymer, nylon 66/PP copolymer, nylon66/PPS copolymer, used alone or in combination thereof.

Preferably, said self-supporting thermoplastic film 11 is of a polyesterselected from: polybutylene terephthalate (PBT), polyethyleneterephthalate (PET), polyethylene isophtalate (PEI), polybutyleneterephthalate/tetramethylene glycol copolymer, PET/PEI copolymer,polyarylate and polybutylene naphthalate. Preferably, theself-supporting thermoplastic film 11 has an elongation at yield greaterthan 5% and preferably not exceeding 30%. Preferably, theself-supporting thermoplastic film 11 has a yield strength evaluatedaccording to the ASTM D882 standard, comprised from about 20 MPa toabout 60 MPa.

Preferably, the self-supporting thermoplastic film 11 has an ultimateelongation greater than 70%. Preferably, the self-supportingthermoplastic film 11 has an ultimate tensile strength comprised fromabout 20 MPa to about 150 MPa. Preferably, in the finished tyre, saidself-supporting thermoplastic film 11 has a thickness “t2” comprisedfrom about 5 μm to about 25 μm. Preferably, in the finished tyre, saidself-supporting thermoplastic film has a resistance to puncturing lowerthan about 30N, preferably lower than about 15N, more preferably lowerthan about 10N.

The layer of sealing polymeric material 10 and the self-supportingthermoplastic film 11 form a sealing assembly 12. The sealing assembly12, when a sharp-pointed element (such as a nail) enters the tyre andpasses through the layer of sealing polymeric material 10 and theself-supporting thermoplastic film 11, is able to adhere to the objectinserted therein and can further flow inside the puncture when saidobject is removed, thereby sealing the puncture itself and preventingescape of air from the tyre. The sealing assembly 12 is easilypierceable by the sharp-pointed element while maintaining adeformability and stickiness capable of helping in transferring thesealing material during ejection of the sharp-pointed element. At thesame time, the thermoplastic film is believed to limit the amount ofsealing material transferred into the puncture or hole so that it doesnot seal holes having sizes greater than a predetermined value, startingfrom holes caused by sharp-pointed elements of an 8 mm diameter, forexample.

Preferably, the tyre further comprises two elongated elements ofelastomeric material 13, each disposed at a circumferential edge of thesealing assembly 12. An axially internal portion 13 a of each elongatedelement of elastomeric material 13 overlaps the sealing assembly 12 andis disposed at a radially internal position to said sealing assembly 12.An axially external portion 13 b of each elongated element ofelastomeric material 13 lies directly in contact with liner 9. Byaxially internal portion 13 a it is intended a portion that is closer toan equatorial plane “X” of tyre 1 than the axially external portion 13b.

In greater detail, the radially internal portion 13 a, in turn, has anaxially internal region directly applied to the self-supportingthermoplastic film 11 and an axially external region directly appliedonto a surface of the layer of sealing polymeric material 10. In fact,the layer of sealing polymeric material 10 has an axial extensiongreater than the axial extension of the self-supporting thermoplasticfilm 11. As a result, each elongated element of elastomeric material 13is directly in contact with both the layer of sealing polymeric material10 and the self-supporting thermoplastic film 11.

Building of a precursor of a green tyre 1 as above described, inclusiveof the sealing assembly 12, is preferably carried out through assemblingof respective semi-finished products on one or more forming supports,not shown.

The carcass structure and belt structure are generally made separatelyfrom each other at respective work stations, to be then mutuallyassembled. In greater detail, manufacture of the carcass structure firstcontemplates formation of the sealing assembly 12 as a continuous ribboncomprising the layer of sealing material 10 disposed on and supported bythe self-supporting thermoplastic film 11 joined to the elongatedelements of elastomeric material 13 associated with oppositelongitudinal edges of said sealing assembly 12 (FIG. 2). Preferably,before embedding of the sealing assembly 12 in the tyre precursor andshaping of same, each elongated element 13 is in direct contact with theself-supporting thermoplastic film 11 over a first width “L1” and withthe layer of sealing material 10 over a second width “L2”. Said widthsare preferably substantially equal to each other.

The sealing layer 10 before embedding of the sealing assembly 12 in thetyre precursor and shaping of same, has a thickness “t3” comprised fromabout 3 mm to about 6 mm.

The self-supporting thermoplastic film 11 before embedding of thesealing assembly 12 in the tyre precursor and shaping of same has athickness “t4” smaller than 50 μm and preferably comprised from about 10μm to about 30 μm.

The sealing assembly 12 provided with the respective elongated elementsof elastomeric material 13 is cut to size and wound up around a radiallyexternal surface of a building drum, the thermoplastic film 11 beingmaintained in a radially innermost position. Opposite end flaps of thesealing assembly 12 are mutually spliced by means of adhesive tape, forexample.

Liner 9 and the carcass ply or plies 3 are applied onto the sealingassembly 12 so as to form a so-called “carcass sleeve”, typically ofsubstantially cylindrical shape. The annular anchoring structures 4 tothe beads 5 are fitted or formed on the opposite end flaps of thecarcass ply or plies 3 that are subsequently turned up around theannular structures 4 so as to enclose them loop-wise.

Simultaneously, on a second drum or auxiliary drum, a so-called “outersleeve” is formed which comprises the belt layers 6 applied in radiallysuperposed relationship with each other, and possibly the tread band 7applied at a radially external position to the belt layer 6. The outersleeve is then picked up from the auxiliary drum to be coupled to thecarcass sleeve. To this aim, the outer sleeve is disposed coaxiallyaround the carcass sleeve and then the carcass ply or plies 3 are shapedinto a toroidal configuration by mutual axial approaching of the beads 5and simultaneous admission of fluid under pressure into the carcasssleeve, so as to determine a radial expansion of the carcass ply 3 untilmaking them adhere against the inner surface of the outer sleeve.

Preferably, after shaping, the thermoplastic film shows a quick stressrelaxation so that an excessive elastic stress is prevented from actingand deforming the green tyre or causing separation of the radially innerlayers thereof. Preferably the self-supporting thermoplastic film hassuch a stress relaxation referred to time, that in the 10 first secondsfollowing achievement of an imposed predetermined elongation thethermoplastic film shows a reduction of at least 20% of the stressnecessary for maintaining such an imposed elongation.

Preferably, the self-supporting thermoplastic film has such a stressrelaxation referred to time that within the 300 seconds followingachievement of an imposed predetermined elongation the thermoplasticfilm shows a reduction of at least 35% of the stress necessary formaintaining such an imposed elongation.

Assembling of the carcass sleeve with the outer sleeve can be carriedout on the same drum used for making the carcass sleeve, in which casethe process is referred to as “unistage building process” or “unistageprocess”.

Also known are the so-called “two-stage” building processes in which aso-called “first-stage drum” is used for making the carcass sleeve,while assembling between the carcass structure and outer sleeve iscarried out on a so-called “second-stage drum” or “shaping drum” ontowhich the carcass sleeve picked up from the first-stage drum and,subsequently, the outer sleeve picked up from the auxiliary drum aretransferred.

After building of the green tyre 1, a curing and molding treatment iscarried out which aims at determining the structural stabilisation oftyre 1 through cross-linking of the elastomeric blends as well asimpressing the tread band 7 with a desired tread pattern and stampingpossible distinctive graphic marks at the sidewalls 8.

During curing, between the elastomer macromolecules a lattice ofcovalent bonds is created that, depending on the density thereof,prevents flowing of the elastomer, making the material increasingly moreinsoluble, infusible and elastic. At all events, after curing, the layerof sealing material 10 keeps its deformability and stickiness features.

EXAMPLE

A Pzero Red 235/45R17 Pirelli tyre was made self-sealing by a sealingassembly comprising a sealing composition together with aself-supporting thermoplastic film of nylon Filmon CXS18 of anon-oriented polyamide 6 having a 18 micron thickness. Forcharacterising the thermoplastic film, a tensile stress test was carriedout on two test pieces of Filmon CXS18 according to the ASTM D882standard, under the following test conditions:

-   -   test temperature 23° C.    -   relative humidity 46%    -   test speed=500 mm/min    -   test-piece length 12.57 mm

The test results are reproduced in the following table 1.

TABLE 1 Ultimate tensile stress Ultimate elongation Sample (Mpa) (%) 153.82 172.48 2 59.58 192.45

A stress relaxation test was also carried out on a sample of FilmonCXS18 that, being measured, had the following sizes 200 mm×20 mm. Thetest piece was conditioned for 24 hours at 23° C. and 45% of relativehumidity.

The test was carried out by a Zwic dynamometer model 1445 and the stressrelaxation values found were the following.

For a test carried out with 130% elongation:

Stress relaxation of 32% after 10 seconds;

Stress relaxation of 48% after 300 seconds;

For a test carried out with 110% elongation:

Stress relaxation of 30% after 10 seconds;

Stress relaxation of 45% after 300 seconds.

Tests were also made for resistance to puncturing on a sample of FilmonCXS 50 of 50 microns. The samples showed values of resistance topuncturing of 9.3N and 22N respectively, measured under the followingconditions:

-   -   T=23° C.    -   relative humidity=50%    -   Diameter of the rounded point=2 mm    -   Test speed=500 mm/min.

The sealing composition of the following table 2 was used for preparingthe sealing layer.

TABLE 2 IR 60 SBR 40 Peptizer 0.5 Process oil 55 Escorez ® 1102 40Struktol ® 40MS 7 N326 15

In the preceding table 2:

-   -   IR is a cis-1,4-polyisoprene elastomer produced by        Nizhnekamskneftechim Export, Russia;    -   SBR is a styrene-butadiene elastomer copolymer cross-linked with        divinyl benzene produced by International Specialty Products        (ISP);    -   Peptizer is PEPTON 66 produced by Anchor Chemical Ltd, UK;    -   Process oil (selected from MES=Mild Extraction Solvates) is a        mineral base oil solvent-refined and/or refined by        hydrotreatment to a high degree (Catenex SNR produced by Shell);    -   Escorez® 1102 is an aliphatic tackiness resin produced by        ExxonMobil;    -   Struktol® 40 MS is a blend of aromatic aliphatic naphthenic        hydrocarbon resins (Struktol Corporation);    -   N326 is carbon black.

The layer of sealing material before building of the tyre had athickness of about 4.0 mm and the sealing assembly was disposed at aradially internal position to the liner (as shown in FIG. 1).

The cured and molded tyres were mounted on a standard rim and inflatedto a 2.4 bar pressure.

Static Sealing Tests

1) Nails of a 3, 4, 5, 8 and 10 mm diameter and 40 mm long were radiallyinserted through the tread of a tyre inflated to a 240 kPa pressure. Thetread region to be punctured corresponded to the belts. The nailarrangement included blocks and grooves and was random on thecircumference.

2) The inserted nails were extracted and the possible air escape wascontrolled using a soapy water solution.

Dynamic Tests

1) Nails of a 3, 4 and 5 mm diameter and 40 mm long were insertedthrough the tread of a tyre inflated to a 240 kPa pressure. The treadregion to be punctured corresponded to the belts. The nail arrangementincluded blocks and grooves and was random on the circumference.

The casing was set in a rolling movement with the inserted nails on aso-called “roller test bench”, a disc of a 2.8 m diameter, at the speedof 120 km/h with a load of 550 kg.

2) First 200 km were covered and then as far as 500 km with a tyre and750 km with another tyre alternating periods of 10 minutes with a zeroslip angle with periods of 10 minutes with a slip angle oscillatingbetween −6° and +6°. The slipping speed was 1°/s==>for each slip cycle25 oscillations were carried out. Air did not escape through the casingsover the whole test period (200, 500 or 750 km).

3) The inserted nails were extracted and 20 km were covered with a loadof 550 kg and a slip angle oscillating between −2° to +2°. The airescape through the punctures was checked with a solution of soapy water.

The obtained results are summarised in the following tables 3 and 4bearing both the number of nails for each diameter and the number ofsealings obtained in percent and as the total number.

TABLE 3 Dynamic Tests diameter of the inserted nails 3 mm 4 mm 5 mmSealing at 200 km No. of the inserted nails 6 4 3 No. of sealedpunctures after 6 4 3 extraction sealing % 100 100 100 Sealing at 500 kmNo. of the inserted nails 10 4 3 No. of sealed punctures after 10 4 2extraction sealing % 100 100 66.7 Sealing at 750 km No. of the insertednails 12 12 12 No. of sealed punctures after 12 10 10 extraction sealing% 100 83.3 83.3

TABLE 4 Static sealing tests with nails of 8 and 10 mm two 8 mmpunctures air escape two 10 mm punctures air escape

Control Test

A P7 235/45R17 Pirelli tyre was made self-sealing by arranging, on thesubstantially air-tight radially innermost layer, a sealing layer of athickness of about 4 mm comprising the sealing composition of said table2 not associated with any film.

The tyre thus made was submitted to a static sealing test. Nails of a 8and 10 mm diameter and 40 mm long were inserted radially, through thetread of a tyre inflated to a pressure of 240 kPa. The tread region tobe punctured corresponded to the belts and arrangement of the nails tookplace in the grooves and was random on the circumference.

The inserted nails were extracted and the possible air escape wascontrolled with a soapy water solution.

The results are reproduced in the following table 5.

TABLE 5 Static sealing tests with nails of 8 and 10 mm on a self-sealing tyre without thermoplastic film two 8 mm punctures No air escapetwo 10 mm punctures No air escape

It was possible to see that the tyre made in accordance with theinvention has allowed a selective control of the sealing even underconditions of greater hard use of the tyre.

In fact the sealing was excluded already in the test carried out understatic conditions, for punctures caused by sharp-pointed elements ofsizes (8 mm and 10 mm) considered as potentially dangerous in accordancewith the Applicant's experiences.

1-25. (canceled)
 26. A method of selectively controlling theself-sealing ability of a tyre comprising: arranging a sealing assemblyon a drum; subsequently, forming a precursor of a green tyre inclusiveof at least one liner and a carcass on said sealing assembly; andsubsequently, shaping, moulding and curing the combined sealing assemblyand tyre precursor, said sealing assembly comprising: i. aself-supporting thermoplastic film; and ii. a layer of sealing materialpossessing viscoelastic and stickiness features, which is associatedwith and supported by said self-supporting thermoplastic film, saidsealing assembly being easily puncturable by a sharp-pointed elementwhile maintaining such a deformability and stickiness that itcontributes to transfer of the sealing material during ejection of thesharp-pointed element and limits the transferred amount thereof to suchan extent that holes bigger than a predetermined value are not sealed.27. The method as claimed in claim 26, wherein said self-supportingthermoplastic film comprises a polyamide selected from: nylon 6, nylon66, nylon 46, nylon 11, nylon 12, nylon 610, nylon 612, nylon 6/66copolymer, nylon 6/66/610 copolymer, nylon MXD 6, nylon 6T, nylon 6/6Tcopolymer, nylon 66/PP copolymer, nylon 66/PPS copolymer, and mixturesthereof.
 28. The method as claimed in claim 26, wherein saidself-supporting thermoplastic film comprises polyester selected from:polybutylene terephthalate, polyethylene terephthalate, polyethyleneisophtalate, polybutylene terephthalate/tetramethylene glycol copolymer,polyethylene terephthalate/polyethylene isophthalate copolymer,polyarylate, and polybutylene naphthalate.
 29. The method as claimed inclaim 26, wherein the self-supporting thermoplastic film has anelongation at yield greater than 5%.
 30. The method as claimed in claim26, wherein the self-supporting thermoplastic film has a yield strengthfrom about 20 MPa to about 60 MPa.
 31. The method as claimed in claim26, wherein the self-supporting thermoplastic film has an ultimateelongation greater than 70%.
 32. The method as claimed in claim 26,wherein the self-supporting thermoplastic film has an ultimate tensilestrength from about 20 MPa to about 150 MPa.
 33. The method as claimedin claim 26, wherein the self-supporting thermoplastic film has such astress relaxation relative to time, that in 10 seconds followingachievement of an imposed predetermined elongation, the thermoplasticfilm shows a reduction of at least 20% of stress necessary formaintaining such an imposed elongation.
 34. The method as claimed inclaim 26, wherein the self-supporting thermoplastic film has such astress relaxation relative to time that within 300 seconds followingachievement of an imposed predetermined elongation, the thermoplasticfilm shows a reduction of at least 35% of stress necessary formaintaining such an imposed elongation.
 35. The method as claimed inclaim 26, wherein said self-supporting thermoplastic film, beforeshaping, has a thickness smaller than 50 μm.
 36. The method as claimedin claim 26, wherein said self-supporting thermoplastic film, beforeshaping, has a thickness from about 10 μm to about 30 μm.
 37. The methodas claimed in claim 26, wherein said layer of sealing material, beforeshaping, has a thickness less than about 6 mm.
 38. The method as claimedin claim 26, wherein said layer of sealing material, before shaping hasa thickness greater than about 3 mm.
 39. The method as claimed in claim26, wherein the sealing material comprises: 40 phr to 80 phr of asynthetic or natural elastomer material; 20 phr to 60 phr of anelastomeric block copolymer, or a styrene-butadiene elastomer; 40 phr to60 phr of process oil; 15 phr to 60 phr of at least one tackiness agent;and 1 phr to 40 phr of at least one reinforcing filler.
 40. The methodas claimed in claim 26, wherein the sealing material comprises: 50 phrto 70 phr of a synthetic or natural elastomer material; 30 phr to 50 phrof an elastomeric block copolymer, or a styrene-butadiene elastomer; 50phr to 60 phr of process oil; 20 phr to 40 phr of at least one tackinessagent; and 5 phr to 30 phr of at least one reinforcing filler.
 41. Themethod as claimed in claim 26, wherein said self-supportingthermoplastic film comprises a polyamide.
 42. The method as claimed inclaim 26, wherein said self-supporting thermoplastic film comprises apolyester.
 43. The method as claimed in claim 26, wherein, after curing,said self-supporting thermoplastic film has a thickness greater thanabout 5 μm and less than about 25 μm, and wherein said sealing layer hasa maximum thickness less than about 6 mm and greater than about 2.5 mm.44. The method as claimed in claim 26, wherein, after curing, saidself-supporting thermoplastic film has a resistance to puncturing lessthan about 30N.
 45. The method as claimed in claim 26, wherein, aftercuring, said self-supporting thermoplastic film has a resistance topuncturing less than about 15N.